This invention relates to coating compositions comprising a mixture of a binder consisting of a mixture of a polyester with an aminoplast and/or a low-molecular weight precursor thereof, water, a monomolecular alcohol as the auxiliary solvent, and, optionally, conventional adjuvants.
Water-dilutable coating compositions containing the aforementioned binders are known (DAS [German Published Application] 1,805,189). This reference also describes the additional use of auxiliary solvents for the purpose of improvement of solubility respectively dilutability.
DOS (German Unexamined Laid-Open Application) 2,253,300 describes monomolecular diols as auxiliary solvents for nonaqueous coating compositions which are low in solvent and/or free of solvent. In this connection, the diol utilized has the effect of a "reactive diluent" since it is chemically incorporated into the binder during the baking step. However, experiments have shown that this incorporation does not take place for such diols in the case of aqueous varnish systems. As a result, the advantages which are derived from such chemical binding of the auxiliary solvent are lost for aqueous systems inasmuch as the organic components, in addition to water, become volatile during the baking step. Consequently, the auxiliary solvent vaporizes causing deleterious environmental air pollution effects.
On the other hand, other types of "reactive diluents" are also known for aqueous systems. For example, DOS 2,439,548 describes monomeric caprolactam as the reactive diluent. However, the nature of the incorporation of this compound into the coating during the baking process is still inadequate.